Method for producing multinary metal oxide powders in a pulsed reactor

ABSTRACT

The invention relates to a method for producing a one-part, multinary metal oxide power which is suitable for producing high-temperature superconductors. To this end, a mixture of the corresponding metal salts and/or metal oxides and/or metals in the requisite stoichiometric ratio is introduced into a pulsed reactor with a pulsed gas flow resulting from flameless combustion and is partially or completely converted into the multinary metal oxide.

[0001] The present invention relates to a method for producing a finelydivided, multinary metal oxide powder, i.e. one which comprises aplurality of elements, which is suitable for use as precursor ofhigh-temperature superconductors.

[0002] High-temperature superconductor materials (HTSCs) are multinaryoxides having high requirements of chemical purity, homogeneity, definedphase composition and crystallite size as well as reproducibility. Theprior art discloses some methods for producing the correspondingmultiphase metal oxide powders from which high-temperaturesuperconductors can be produced by further processing, for example bulkmaterial by pressing, extrusion and/or sintering, or wire and bandconductors by means of “powder-in-tube methods”.

[0003] EP 117 059, EP 522 575, EP 285 392, EP 302 830, EP 912 450 andU.S. Pat. No. 5,298,654 describe the co-precipitation of metalcompounds, for example nitrates or chlorides, dissolved in water. Thewater-insoluble or sparingly soluble metal oxalate mixtures areprecipitated here from the solutions with oxalic acid. Industrialimplementation of these processes requires high technical complexityboth in the co-precipitation and in the disposal or reuse of theresultant degradation products. Spray-drying of co-precipitationproducts of this type, which are usually only present as solid in lowconcentration in the “mother liquor” (order of magnitude of 10%), isunfavourable from an energy point of view. In addition, volatileproducts can be formed by reverse reactions in the thermal dewateringwhich are discharged with the exhaust air and which thus influence thechemical composition of the subsequent products in a non-reproduciblemanner.

[0004] In other methods, mixtures of aqueous salt solutions of theelements which are to be present in the superconductor are subjected tospray pyrolysis.

[0005] WO 89/02871 describes a method for producing multielement metaloxide powders for use as precursors for HTSC ceramics, in which metalmixed-salt solutions are sprayed into a tubular furnace heated to atemperature of 800-1100° C. In this method, on the one hand the externalelectric heating of the tubular furnace means that only a low energyyield is achieved and on the other hand only a relatively low conversionto the mixed oxide is achieved.

[0006] EP 371 211 claims a spray pyrolysis method for producing finelydivided, homogeneous ceramic powders in which a solution or suspensionof compounds which comprise the elements of the powder to be produced issprayed with the aid of a combustible gas, preferably hydrogen gas, intoa reactor in which the gas is burned. Conversion of the sprayed dropletsinto the oxide powders takes place in the flame at a measurabletemperature of 1200-1300° C. In the case of the use of nitratesolutions, it must be assumed that much higher temperatures act on thedroplet/particle system. The production of powders for high-temperaturesuperconductors having a defined composition is difficult with thismethod since the powders generally also comprise volatile metal oxides,for example Bi or Pb oxides, which volatilise in variable proportions.

[0007] DE 195 05 133 describes a method for producing highly disperseoxidic powders in which an aerosol of dissolved or liquid compounds inoxygen is fed to a cracking gas reactor under pressure beforecombustion. Very high reaction temperatures are likewise required here.

[0008] EP 681 989 describes a method in which aerosols of aqueoussolutions comprising a mixture of the corresponding metal salts in therequisite stoichiometric ratio are pyrolysed in a hydrogen/oxygen flame.The flame temperature here is kept in the range from 800 to 1100° C.Contact of the aerosols and the powders produced in the method withcarbon or carbon-containing compounds or materials must be avoided here.

[0009] In summary, it must be noted that the known spray pyrolysismethods for producing high-temperature superconductor powders have thefollowing disadvantages: undesired high-temperature phases form. Theevaporation of volatile oxides can result in stoichiometry variations orin inadequate reproducibility of the chemical composition. The methodsresult in inadequate conversion to the mixed oxide or in a high residualnitrate content. Coarse and hard agglomerates form and pulverulentdeposits form on the reactor wall and have to be removed regularly,which may cause operation of the plants to be interrupted, and are oftenthe source of hard agglomerates. Furthermore, the limited reactorgeometry means that there is only an inadequate possibility of scale-up.

[0010] DD 245 674 and DD 245 649 describe methods for producing silicatesubstances or single-phase oxides in which liquid silica sols or liquidor dissolved metal compounds with organic ligands are atomised andthermally treated in pulsed combustion in a pulsed furnace reactor. Thismethod produces highly disperse silica gels or oxides with specificparticle sizes, surface areas and surface structures.

[0011] The object of the present invention is to provide a technicallyand economically advantageous method for producing a multinary, at leastternary metal oxide powder which is suitable for use as precursor ofhigh-temperature superconductors and which does not have thedisadvantages of the prior art.

[0012] This object is achieved in accordance with the invention by amethod in which a mixture of the corresponding metal salts and/or metaloxides and/or metals in solid form or in the form of a solution orsuspension in the requisite stoichiometric ratio comprising at leastthree elements selected from Cu, Bi, Pb, Y, Tl, Hg, La, lanthanides andalkaline earth metals is introduced into a pulsed reactor with a pulsedgas flow resulting from flameless combustion and is partially orcompletely converted into the multinary metal oxide.

[0013] The method according to the invention is distinguished by thefact that a mixture of the corresponding metal salts, metal oxides ormetals is introduced into a pulsed reactor and is converted into themultinary metal oxide, i.e. one comprising a plurality of elements. Themetals or metal compounds here are employed in the requisitestoichiometric ratio and comprise at least three elements, preferablythree, four or five elements, selected from the group consisting of Cu,Bi, Pb, Y, Tl, Hg, La, lanthanides and alkaline earth metals. Theresultant finely divided metal oxide powder comprising a plurality ofelements is suitable for use in the production of high-temperaturesuperconductor ceramics.

[0014] Surprisingly, in spite of extremely short residence times in thereactor, the method according to the invention gives multinary oxideshaving a high degree of conversion. The method exhibits high reactivityfor target-phase formation and good reproducibility of the composition.The particles produced in accordance with the invention have smallparticle sizes, and no coarse or hard agglomerates are formed during theproduction.

[0015] The principle on which the pulsed reactor works is the same asthat of an acoustic cavity resonator, which consists of a combustionchamber, a resonance tube and a cyclone or filter for powder separation.A pulsed reactor according to the invention is depicted in FIG. 1. Itconsists of a combustion chamber (1), to which a resonance tube (2)having a significantly reduced flow cross section compared with thecombustion chamber is connected on the exhaust-gas side. The combustionchamber base is fitted with one or more valves for the entry of thecombustion gases. The powders are separated from the gas flow using asuitable filter (3) for ultra-fine particles.

[0016] The combustion-gas mixture entering the combustion chamber isignited, burns very quickly and produces a pressure wave in thedirection of the resonance tube since the gas entry side issubstantially sealed in the case of an excess pressure by aerodynamicvalves. The gas flowing out into the resonance tube produces a vacuum inthe combustion chamber, meaning that fresh gas mixture flows in throughthe valves and ignites itself. This process of valve closing and openingdue to pressure and vacuum takes place periodically with selfregulation. The pulsed flameless combustion process in the combustionchamber liberates energy with the propagation of a pressure wave in theresonance tube and initiates an acoustic vibration there. Pulsed flowsof this type are characterised by a high degree of turbulence. Thepulsation frequency can be set via the reactor geometry and varied in atargeted manner via the temperature. This causes the person skilled inthe art no difficulties at all. The gas flow resulting from flamelesscombustion preferably pulses at from 20 to 150 Hz, particularlypreferably at from 30 to 70 Hz.

[0017] With respect to the combustion-chamber pressure and the gasvelocity in the resonance tube, non-steady-state conditions exist,ensuring particularly intense heat transfer, i.e. very fast andextensive energy transfer from the pulsed hot gas flow to the solidparticles. Very considerable reaction progress is thus achieved inaccordance with the invention at very short residence times in themillisecond range. Surprisingly, a high degree of the defined mixedoxide formation can be achieved under these conditions, even in the caseof multinary substance systems. Advantageously, scale-up of the methodaccording to the invention is possible.

[0018] A suitable fuel gas is in principle any gas which is suitable forthe production of hot gas. If desired, this is employed in the form of amixture with oxygen. Natural gas and/or hydrogen mixed with air or, ifdesired, oxygen is preferably used. However, propane or butane, forexample, are also conceivable. In contrast to pyrolysis methods, forexample in a permeation reactor, the combustion air thus also serves ascarrier gas for material transport in the reactor.

[0019] The mixture of the corresponding metals and/or metal compoundscan be introduced into the reactor either in solid form, in particularin the form of a powder, or in the form of a solution or suspension.Solid substance mixtures to be calcined can be conveyed into the gasflow by means of an injector, with the pulsed turbulent flow resultingin fine distribution of the material in the reaction space. Solutions orsuspensions are introduced in extremely finely divided form by means ofone or more nozzles, preferably by means of a two-component nozzle. Veryrapid water removal or thermal decomposition of the reactants thusoccurs, and the solid particles which remain can react in the hot gasflow to form the mixed oxide.

[0020] In a preferred embodiment of the method according to theinvention, the mixture is introduced in the form of an aqueous saltsolution or suspension of nitrates, acetates, citrates, lactates,tartrates, chlorides, hydroxides, carbonates and/or oxalates of thecorresponding metals. Particular preference is given to the use of saltsolutions of the corresponding metals with the same counteranion.

[0021] In accordance with the invention, the mixture is introduced intothe hot gas flow of the pulsed reactor resulting from flamelesscombustion. This causes any solvent present to evaporate or burn, andmetal-salt or metal-oxide particles form and are then convertedcompletely or partially into the multinary metal oxide during thefurther course of the reaction through thermal conversion, oxidationand/or reduction.

[0022] In the method according to the invention, the mixture can beintroduced either directly into the combustion chamber of the pulsedreactor or into the pulsed reactor resonance tube connected to thecombustion chamber. Introduction into the resonance tube has theadvantage that the combustion process is separated from the chemicalsolid-state reactions.

[0023] The flameless combustion and the turbulent flow conditions meanthat a homogeneous temperature distribution exists in the reactionspace, meaning that the raw materials introduced are subject toidentical thermal treatment. Local overheating and wall deposits, whichresult in the formation of coarse and hard agglomerates in the spraypyrolysis methods, are thus avoided. The gas flow resulting from thepulsed combustion in the pulsed reactor has flow turbulences whosedegree of turbulence is, in a preferred embodiment, from 5 to 10 timesgreater than the degree of turbulence of steady-state flow. Thetemperature of the gas flow in the combustion chamber of the pulsedreactor is preferably above 650° C., in particular above 800° C. If thecombustion chamber and, if desired, the resonance tube have ceramiclinings, it is also possible to carry out the method according to theinvention at very high gas-flow temperatures, which cannot be achievedusing other methods.

[0024] The particles produced in the reactor are separated from the gasflow in a suitable separator, the choice of which presents the personskilled in the art with no difficulties at all, such as, for example, agas cyclone, a surface filter or an electrostatic filter.

[0025] The reaction gas is cooled to the temperature necessary dependingon the filter type before entering the separator. This is carried out bymeans of a heat exchanger and/or by introducing cooling gases into theexhaust-gas flow. The particular advantage of the method according tothe invention is that inexpensive high-performance suspended-matterfilters with comparatively large specific filter surface areas andthroughput efficiencies can be used instead of hot-gas filters. Theintroduction of CO₂-free cooling gases enables the production of powdershaving a particularly low residual carbon content. By varying the oxygenpartial pressure during introduction of the cooling gases, the phasecomposition of the powder can be influenced.

[0026] In a variant of the method according to the invention, themixture of the corresponding metals or metal compounds employed mayadditionally comprise dopants in the form of dissolved salts and/ordispersed solids. These dopants are added to the mixture in smallamounts, i.e. up to a maximum of 5% by weight, preferably up to 1% byweight of the mixture, in order specifically to influence certainproperties of the multinary metal oxide powder to be produced. Forexample, dopants enable the crystallite size of secondary phases, whichact as pinning centres, to be limited or the mechanical properties ofthe bulk material to be improved. The term crystallite size is taken tomean the size of the crystallographically uniform region of a powderparticle, and a pinning centre is a centre of adhesion for the magneticflux in superconductors (for example on non-superconducting secondaryphases). The dopant used is one or more of the elements selected fromgroup Ib, for example Ag, from group IIb, for example Zn, from groupIVa, for example Sn, from group IVb, for example Zr, and/or from groupVIIb of the Periodic Table, for example Mn.

[0027] In a further variant of the method according to the invention,the metal oxide powder can be subjected to thermal aftertreatment at atemperature in the range from 500 to 960° C., preferably from 550 to800° C., after the reaction in the pulsed reactor. The choice of asuitable type of post-calcination depending on the powder type, desiredphase composition and application presents the person skilled in the artwith no difficulties at all. Particular preference is given topost-calcination in a powder bed in a chamber, tubular, tunnel, belt orrotary tube furnace or in a fluidised bed. The conditions here should beset in such a way that firstly the desired phase composition is reached,but secondly no formation of hard agglomerates due to sintering ormelting of primary crystallites occurs. If necessary, the powder issubjected to grinding by means of an air-jet mill, grinding-media mill,impact mill or other milling machines.

[0028] Preference is given to methods according to the invention inwhich the corresponding metals or metal compounds are selected from oneof the following compositions: Bi-EA-Cu, (Bi,Pb)-EA-Cu, Y-EA-Cu,(Y,SE)-EA-Cu, Tl-EA-Cu, (Tl,Pb)-EA-Cu or Tl-(Y,EA)-Cu, where EA denotesalkaline earth metal elements, in particular Ba, Ca and/or Sr, and SEdenotes rare-earth metals.

[0029] Particular preference is given to the use in the method accordingto the invention of mixtures in which the substances employed have thefollowing molar ratios of the corresponding metals:

Bi_((2.0±x))Sr_((2.0±x))Ca_((1.0±x))Cu_((2.0±x)) where x=0.3, preferablywhere x=0.2, orPb_((0.3±y))Bi_((1.7±y))Sr_((2.0±y))Ca_((2.0±y))Cu_((3.0±y)) where y=0.3Y_(c)Ba_(d)Cu₃

where 1<c<1.8 and 1.5<d<2.5.

[0030] The present invention furthermore relates to a finely divided,multinary metal oxide powder which has been produced by a methodaccording to the invention. In a particularly preferred embodiment, themean crystallite size of the metal oxide powder produced in accordancewith the invention, i.e. the mean size of the crystallographicallyuniform region of a powder particle, is <500 nm. Preference is given tothe production in accordance with the invention of metal oxide powderswhich consist of one of the following compositions: Bi-EA-Cu-O,(Bi,Pb)-EA-Cu-O, Y-EA-Cu-O, (Y,SE)EA-Cu-O, Tl-EA-Cu-O, (Tl,Pb)-EA-Cu-Oor Tl-(Y,EA)-Cu-O, where EA denotes alkaline earth metal elements, inparticular Ba, Ca and/or Sr, and SE denotes rare-earth metals.

[0031] The present invention likewise relates to the use of the metaloxide powders produced in accordance with the invention for theproduction of high-temperature superconductors.

[0032] The metal oxide powders produced in accordance with the inventioncan be used, for example, for the production ofhigh-temperature-superconducting hollow or solid articles in the form ofsheets, discs, rings, tubes, rods, etc., which can be used as powersupply or bearing components. Powders or pressed rods can be used forthe production of silver-clad high-temperature-superconducting wires orstrip conductors. The wires and strip conductors are used, for example,for power cables, power lines, transformers, motor and generator coils,magnets, power supplies or bearings. Furthermore, the metal mixed oxidepowders produced in accordance with the invention can be used for theproduction of targets for coating methods or can be used for theproduction of coated strip conductors.

[0033] The complete disclosure content of all applications, patents andpublications mentioned above and below and of the correspondingapplication DE 101 11 938.0, filed on 13 Mar. 2001, are incorporatedinto this application by way of reference.

[0034] Even without further details, it is assumed that a person skilledin the art will be able to utilise the above description in its broadestscope. The preferred embodiments and examples of the method according tothe invention should therefore merely be regarded as descriptivedisclosure which is absolutely not limiting in any way.

EXAMPLE 1

[0035] A mixture of aqueous nitrate solutions of the elements Bi, Pb,Sr, Ca and Cu in accordance with the stoichiometryBi_(1.75)Pb_(0.35)Sr_(1.98)Ca_(2.0)CU₃O_(x) is produced, with the totalsalt content of the mixed nitrate solution being 40%.

[0036] The geometry of the pulsed reactor is defined by the combustionchamber length to combustion chamber diameter ratio of 2.2 and by theresonance tube length to resonance tube diameter ratio of 33. The mixednitrate solution is introduced into the front section of the resonancetube in the form of an aerosol by means of a two-component nozzle. Theprocess parameters of amount of fuel (hydrogen) V_(H2) and amount ofcombustion air V_(VL) are selected in accordance with the mixed nitratesolution sprayed in M in such a way that the desired reactiontemperature of 700° C. becomes established in the resonance tube:V_(H2)=2.5 kg/h; V_(VL)=195 kg/h; M=10 kg/h. The combustion gases at theend of the resonance tube comprise 16.9% of O₂, 0.09% of CO₂ and 0.24%of NO.

[0037] Powder separation is carried out by means of cassette filtershaving a filter area of 24 m² and a maximum surface temperature of 130°C.

[0038] Properties of the Bi_(1.75)Pb_(0.35)Sr_(1.98)Ca_(2.0)CU₃O_(x)powder produced:

[0039] mean particle size 0.15 μm

[0040] specific surface area 9.4 m²/g

[0041] residual nitrate content: 6.0%

[0042] Post-calcination is carried out for 8 hours in a chamber furnaceat a temperature of 810° C., with the multinary metal oxide powder beingintroduced into Ag boats with a maximum bed depth of 4 cm. The nitratecontent of the samples is reduced to values <100 ppm, and the desiredphase composition of the multinary metal oxide powder produced is set.

EXAMPLE 2

[0043] A substance mixture as described in Example 1 is sprayed axiallyinto the combustion chamber of the pulsed reactor. With V_(H2)=3.1 kg/h;V_(VL)=195 kg/h; M=10 kg/h, a reactor temperature of 900° C. is set. Thecombustion gases comprise 14.6% of O₂, 0.08% of CO₂ and 0.28% of NO. Allother parameters correspond to those of Example 1.

[0044] Properties of the Bi_(1.75)Pb_(0.35)Sr_(1.98)Ca_(2.0)CU₃O_(x)powder produced:

[0045] mean particle size 0.24 μm

[0046] specific surface area 8.4 m²/g

[0047] residual nitrate content: 4.4%

[0048] Post-calcination for 8 hours in a chamber furnace at atemperature of 800° C., with the multinary metal oxide powder beingintroduced into Ag boats with a maximum bed depth of 4 cm, reduces thenitrate content of the samples to values <100 ppm, and the desired phasecomposition of the multinary metal oxide powder produced is set.

EXAMPLE 3

[0049] A mixture of chlorides of the elements Y, Ba and Cu correspondingto the stoichiometric ratio Y_(1.5)Ba₂CU₃ is sprayed into the combustionchamber of the reactor as described in Example 1. With V_(H2)=1.0 kg/h;V_(VL)=75 kg/h; M=3.0 kg/h, a reactor temperature of 900° C. is set.

[0050] Powder properties:

[0051] mean particle size 70 nm

[0052] specific surface area 12 m²/g

[0053] residual chloride content: 2.5%

[0054] The post-calcination is carried out for 4 hours at 710° C. in achamber furnace. This causes the residual chloride content to be reducedto <50 ppm without reducing the sinter reactivity of the powder.

1. Method for producing a finely divided, multinary metal oxide powderwhich is suitable for use as precursor of high-temperaturesuperconductors, characterised in that a mixture of the correspondingmetal salts and/or metal oxides and/or metals in solid form or in theform of a solution or suspension in the requisite stoichiometric ratiocomprising at least three elements selected from Cu, Bi, Pb, Y, TI, Hg,La, lanthanides and alkaline earth metals, is introduced into a pulsedreactor with a pulsed gas flow resulting from flameless combustion andis partially or completely converted into the multinary metal oxide. 2.Method according to claim 1, characterised in that the mixtureintroduced is an aqueous salt solution or suspension of nitrates,acetates, citrates, lactates, tartrates, chlorides, hydroxides,carbonates and/or oxalates.
 3. Method according to at least one of thepreceding claims, characterised in that the mixture is introduceddirectly into the combustion chamber of the pulsed reactor or into thepulsed reactor resonance tube connected to the combustion chamber. 4.Method according to at least one of the preceding claims, characterisedin that the gas flow in the pulsed reactor resulting from flamelesscombustion pulses at from 20 to 150 Hz, in particular at from 30 to 70Hz.
 5. Method according to at least one of the preceding claims,characterised in that the gas flow in the pulsed reactor resulting fromflameless combustion has flow turbulences whose degree of turbulence is5-10 times greater than that of steady-state flow.
 6. Method accordingto at least one of the preceding claims, characterised in that the gasflow in the combustion chamber of the pulsed reactor has temperaturesabove 650° C., in particular above 800° C.
 7. Method according to atleast one of the preceding claims, characterised in that the mixturecomprises dopants in the form of dissolved salts and/or dispersed solidsof one or more of the elements selected from group Ib of the PeriodicTable, in particular Ag, group IIb, in particular Zn, group IVa, inparticular Sn, group IVb, in particular Zr, and group VIIb, inparticular Mn.
 8. Method according to at least one of the precedingclaims, characterised in that the metal oxide powder is subjected tothermal aftertreatment at a temperature in the range from 500 to 960°C., preferably from 550 to 800° C., after the reaction in the pulsedreactor.
 9. Method according to claim 8, characterised in that thethermal after-treatment is carried out in a chamber, tubular, tunnel,belt or rotary tube furnace or in a fluidised bed reactor.
 10. Methodaccording to at least one of the preceding claims, characterised in thatthe mixture employed comprises compounds with elements selected from thefollowing groups: Bi-EA-Cu, (Bi,Pb)-EA-Cu, Y-EA-Cu, (Y,SE)-EA-Cu,Tl-EA-Cu, (Tl,Pb)-EA-Cu or Tl-(Y,EA)-Cu, where EA denotes alkaline earthmetal elements, in particular Ba, Ca and/or Sr, and SE denotesrare-earth metals.
 11. Method according to claim 10, characterised inthat the compounds of the mixture are employed in the following molarratio of the elements Bi_((2.0±x))Sr_((2.0±x))Ca_((1.0±x))Cu_((2.0±x))where x=0.3, preferably where x=0.2, orPb_((0.3±y))Bi_((1.7±y))Sr_((2.0±y))Ca_((2.0±y))Cu_((3.0±y)) where y=0.3or Y_(c)Ba_(d)Cu₃ where 1<c<1.8 and 1.5<d<2.5.
 12. Method according toat least one of the preceding claims, characterised in that the finelydivided metal oxide powder produced has a mean crystallite size of <500nm.
 13. Finely divided, multinary metal oxide powders which have beenproduced by a method according to one of claims 1 to
 12. 14. Metal oxidepowders according to claim 13, characterised in that they consist of oneof the following compositions: Bi-EA-Cu-O, (Bi,Pb)-EACu-O, Y-EA-Cu-O,(Y,SE)-EA-Cu-O, Tl-EA-Cu-O, (Tl,Pb)-EA-Cu-O or Tl-(Y,EA)-Cu-O, where EAdenotes alkaline earth metal elements, in particular Ba, Ca and/or Sr,and SE denotes rare-earth metals.
 15. Use of the metal oxide powderproduced according to one or more of claims 1 to 12 for the productionof high-temperature superconductors.